A) Lipids
Lipids are water-insoluble organic biomolecules, which are essential components of diverse biological functions, including the storage, transport, and metabolism of energy, and membrane structure and fluidity. Lipids are derived from two sources in man and other animals: some lipids are ingested as dietary fats and oils and other lipids are biosynthesized by the human or animal. In mammals at least 10% of the body weight is lipid, the bulk of which is in the form of triacylglycerols.
Triacylglycerols, also known as triglycerides and triacylglycerides, are made up of three fatty acids esterified to glycerol. Dietary triacylglycerols are stored in adipose tissues as a source of energy, or hydrolyzed in the digestive tract by triacylglycerol lipases, the most important of which is pancreatic lipase. Triacylglycerols are transported between tissues in the form of lipoproteins.
Lipoproteins are micelle-like assemblies found in plasma which contain varying proportions of different types of lipids and proteins (called apoproteins). There are five main classes of plasma lipoproteins, the major function of which is lipid transport. These classes are, in order of increasing density, chylomicrons, very low density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low density lipoproteins (LDL), and high density lipoproteins (HDL). Although many types of lipid are found associated with each lipoprotein class, each class transports predominantly one type of lipid: triacylglycerols described above are transported in chylomicrons, VLDL, and IDL; while phospholipids and cholesterol esters are transported in HDL and LDL respectively.
Phospholipids are di-fatty acid esters of glycerol phosphate, also containing a polar group coupled to the phosphate. Phospholipids are important structural components of cellular membranes. Phospholipids are hydrolyzed by enzymes called phospholipases. Phosphatidylcholine, an exemplary phospholipid, is a major component of most eukaryotic cell membranes.
Cholesterol is the metabolic precursor of steroid hormones and bile acids as well as an essential constituent of cell membranes. In man and other animals, cholesterol is ingested in the diet and also synthesized by the liver and other tissues. Cholesterol is transported between tissues in the form of cholesteryl esters in LDLs and other lipoproteins.
Membranes surround every living cell, and serve as a barrier between the intracellular and extracellular compartments. Membranes also enclose the eukaryotic nucleus, make up the endoplasmic reticulum, and serve specialized functions such as in the myelin sheath that surrounds axons. A typical membrane contains about 40% lipid and 60% protein, but there is considerable variation. The major lipid components are phospholipids, specifically phosphatidylcholine and phosphatidylethanolamine, and cholesterol. The physicochemical properties of membranes, such as fluidity, can be changed by modification of either the fatty acid profiles of the phospholipids or the cholesterol content. Modulating the composition and organization of membrane lipids also modulates membrane-dependent cellular functions, such as receptor activity, endocytosis, and cholesterol flux.
B) Enzymes
The triacylglycerol lipases are a family of enzymes which play several pivotal roles in the metabolism of lipids in the body. Three members of the human triacylglycerol lipase family have been described: pancreatic lipase, lipoprotein lipase, and hepatic lipase (Goldberg, I. J., Le, N.-A., Ginsberg, H. N., Krauss, R. M., and Lindgren, F. T. (1988) J. Clin. Invest. 81, 561–568; Goldberg, I. J., Le, N., Paterniti J. R., Ginsberg, H. N., Lindgren, F. T., and Brown, W. V. (1982) J. Clin. Invest. 70, 1184–1192; Hide, W. A, Chan, L., and Li, W.-H. (1992) J. Lipid. Res. 33, 167–178). Pancreatic lipase is primarily responsible for the hydrolysis of dietary lipids. Variants of pancreatic lipase have been described, but their physiological role has not been determined (Giller, T., Buchwald, P., Blum-Kaelin, D., and Hunziker, W. (1992) J. Biol. Chem. 267, 16509–16516). Lipoprotein lipase is the major enzyme responsible for the distribution and utilization of triglycerides in the body. Lipoprotein lipase hydrolyzes triglycerides in both chylormicrons and VLDL. Hepatic lipase hydrolyzes triglycerides in IDL and HDL, and is responsible for lipoprotein remodeling. Hepatic lipase also functions as a phospholipase, and hydrolyzes phospholipids in HDL.
Phospholipases play important roles in the catabolism and remodeling of the phospholipid component of lipoproteins and the phospholipids of membranes. Phospholipases also play a role in the release of arachidonic acid and the subsequent formation of prostaglandins, leukotrienes, and other lipids which are involved in a variety of inflammatory processes.
The lipase polypeptides encoded by these lipase genes are approximately 450 amino acids in length with leader signal peptides to facilitate secretion. The lipase proteins are comprised of two principal domains (Winkler, K., D'Arcy, A., and Hunziker, W. (1990) Nature 343, 771–774). The amino terminal domain contains the catalytic site while the carboxyl domain is believed to be responsible for substrate binding, cofactor association, and interaction with cell receptors (Wong, H., Davis, R. C., Nikazy, J., Seebart, K. E., and Schotz, M. C. (1991) Proc. Natl. Acad. Sci. USA 88, 11290–11294; van Tilbeurgh, H., Roussel, A, Lalouel, J.-M., and Cambillau, C. (1994) J. Biol. Chem. 269, 4626–4633; Wong, H., Davis, R. C., Thuren, T., Goers, J. W., Nikazy, J., Waite, M.. and Schotz, M. C. (1994) J. Biol. Chem. 269, 10319–10323; Chappell, D. A., Inoue, I., Fry, G. L., Pladet, M. W., Bowen, S. L., Iverius, P.-H., Lalouel, J.-M., and Strickland, D. K. (1994) J. Biol. Chem. 269,18001–18006). The overall level of amino acid homology between members of the family is 22–65%, with local regions of high homology corresponding to structural homologies which are linked to enzymatic function.
The naturally occurring lipoprotein lipase protein is glycosylated, and glycosylation is necessary for LPL enzymatic activity (Semenkovich, C. F., Luo, C.-C., Nakanishi, M. K, Chen, S.-H., Smith, L C., and Chan L (1990) J. Biol. Chem. 265, 5429–5433). There are two sites for N-linked glycosylation in hepatic and lipoprotein lipase and one in pancreatic lipase. Additionally, four sets of cysteines form disulfide bridges which are essential in maintaining structural integrity for enzymatic activity (Lo, J.-Y., Smith, L. C., and Chan, L. (1995) Biochem. Biophys. Res. Commun. 206, 266–271; Brady, L., Brzozowski, A. M., Derewenda, Z. S., Dodson, E., Dodson G., Tolley, S., Turkenburg, J. P., Christiansen, L., Huge-Jensen B., Norskov, L., Thim, L., and Menge, U. (1990) Nature 343, 767–770).
Members of the triacylglycerol lipase family share a number of conserved structural features. One such feature is the “GXSXG” motif, in which the central serine residue is one of the three residues comprising the “catalytic triad” (Winkler, K., D'Arcy, A., and Hunziker, W. (1990) Nature 343, 771–774; Faustinella, F., Smith, L. C., and Chan, L. (1992) Biochemistry 31,7219–7223). Conserved aspartate and histidine residues make up the balance of the catalytic triad. A short span of 19–23 amino acids (the “lid region”) forms an amphipathic helix structure and covers the catalytic pocket of the enzyme (Winkler, K, D'Arcy, A., and Hunziker, W. (1990) Nature 343, 771–774). This region diverges between members of the family, and it has recently been determined that the span confers substrate specificity to the enzymes (Dugi, K. A., Dichek H. L., and Santamarina-Fojo, S. (1995) J. Biol. Chem. 270, 25396–25401). Comparisons between hepatic and lipoprotein lipase have demonstrated that differences in triacylglycerol lipase and phosphoilpase activities of the enzymes are in part mediated by this lid region (Dugi, K. A., Dichek H. L., and Santamarina-Fojo, S. (1995) J. Biol. Chem. 270, 25396–25401).
The triacylglycerol lipases possess varying degrees of heparin binding activity. Lipoprotein lipase has the highest affinity for heparin, and this binding activity has been mapped to stretches of positively charged residues in the amino terminal domain (Ma, Y., Henderson, H. E., Liu, M.-S., Zhang, H., Forsythe, I. J., Clarke-Lewis, I., Hayden, M. R., and Brunzell, J. D. J. Lipid Res. 35, 2049–2059). The localization of lipoprotein lipase to the endothelial surface (Cheng, C. F., Oosta, G. M., Bensadoun, A., and Rosenberg, R. D. (1981) J. Biol. Chem. 256, 12893–12896) is primarily mediated through binding to surface proteoglycans (Shimada K., Gill, P. J., Silbert, J. E., Douglas, W. H. J., and Fanburg, B. L. (1981) J. Clin. Invest. 68, 995–1002; Saxena, U., Klein, M. G., and Goldberg, I. J. (1991) J. Biol. Chem. 266, 17516–17521; Eisenberg, S., Sehayek, E., Olivecrona, T., and Vlodavsky, I. (1992) J. Clin Invest. 90,2013–2021). It is this binding activity which allows the enzyme to accelerate LDL uptake by acting as a bridge between LDL and the cell surface (Mulder, M., Lombardi, P., Jansen, H., vanBerkel T. J., Frants R. R., and Havekes, L. M. (1992) Biochem. Biophys. Res. Comm. 185, 582–587; Rutledge, J. C., and Goldberg, I. J., (1994) J. Lipid Res. 35. 1152–1160; Tsuchiya, S., Yamabe, M., Yamaguchi, T., Kobayashi, Y., Konno, T., and Tada, K (1980) Int. J. Cancer 26,171–176).
Lipoprotein lipase and hepatic lipase are both known to function in conjunction with co-activator proteins: apolipoprotein CII for lipoprotein lipase and colipase for pancreatic lipase.
The genetic sequences encoding human pancreatic lipase, hepatic lipase and lipoprotein lipase have been reported (Genbank accession #M93285, #J03540, and #M15856 respectively). The messenger RNAs of human hepatic lipase and pancreatic lipase are approximately 1.7 and 1.8 kilobases in length respectively. Two mRNA transcripts of 3.6 and 3.2 kilobases are produced from the human lipoprotein lipase gene. These two transcripts utilize alternate polyadenylation signals, and differ in their translational efficiency (Ranganathan, G., Ong, J. M., Yukht, A., Saghizadeh, M., Simsolo, R. B., Pauer, A, and Kern, P. A. (1995) J. Biol. Chem. 270, 7149–7155).
C) Physiological Processes
The metabolism of lipids involves the interaction of lipids, apoproteins, lipoproteins, and enzymes.
Hepatic lipase and lipoprotein lipase are multifunctional proteins which mediate the binding, uptake, catabolism, and remodeling of lipoproteins and phospholipids. Lipoprotein lipase and hepatic lipase function while bound to the luminal surface of endothelial cells in peripheral tissues and the liver respectively. Both enzymes participate in reverse cholesterol transport, which is the movement of cholesterol from peripheral tissues to the liver either for excretion from the body or for recycling. Genetic defects in both hepatic lipase and lipoprotein lipase are known to be the cause of familial disorders of lipoprotein metabolism. Defects in the metabolism of lipoproteins result in serious metabolic disorders, including hypercholesterolemia, hyperlipidemia, and atherosclerosis.
Atherosclerosis is a complex, polygenic disease which is defined in histological terms by deposits (lipid or fibrolipid plaques) of lipids and of other blood derivatives in blood vessel walls, especially the large arteries (aorta, coronary arteries, carotid). These plaques, which are more or less calcified according to the degree of progression of the atherosclerotic process, may be coupled with lesions and are associated with the accumulation in the vessels of fatty deposits consisting essentially of cholesterol esters. These plaques are accompanied by a thickening of the vessel wall, hypertrophy of the smooth muscle, appearance of foam cells (lipid-laden cells resulting from uncontrolled uptake of cholesterol by recruited macrophages) and accumulation of fibrous tissue. The atheromatous plaque protrudes markedly from the wall, endowing it with a stenosing character responsible for vascular occlusions by atheroma, thrombosis or embolism, which occur in those patients who are most affected. These lesions can lead to serious cardiovascular pathologies such as infarction, sudden death, cardiac insufficiency, and stroke.
The role of triacylglycerol lipases in vascular pathologies such as atherosclerosis has been an area of intense study (reviewed in Olivecrona, G., and Olivecrona, T. (1995) Curr. Opin. Lipid. 6,291–305). Generally, the action of the triacylglycerol lipases is believed to be antiatherogenic because these enzymes lower serum triacylglycerol levels and promote HDL formation. Transgenic animals expressing human lipoprotein lipase or hepatic lipase have decreased levels of plasma triglycerides and an increased level of high density lipoprotein (HDL) (Shimada, M., Shimano, H., Gotoda, T., Yamamoto, K., Kawamura, M., Inaba, T., Yazaki, t., and Yamada, N. (1993) J. Biol. Chem. 268, 17924–17929; Liu, M.-S., Jirik, F. R., LeBoeuf, R. C., Henderson, H., Castellani, L. W., Lusis, A. J., ma, Y., Forsythe, I. J., Zhang, H., Kirk, E., Brunzell, J. D., and Hayden, M. R. (1994) J. Biol. Chem. 269, 11417–11424). Humans with genetic defects resulting in decreased levels of lipoprotein lipase activity have been found to have hypertriglyceridemia but no increased risk of coronary heart disease. This is reported to be due to the lack of production of intermediate-sized, atherogenic lipoproteins which could accumulate within the subendothelial space (Zilversmit, D. B. (1973) Circ. Res. 33, 633–638).
In the localized area of an atherosclerotic lesion, however, the increased level of lipase activity is hypothesized to accelerate the atherogenic process (Zilversmit, D. B. (1995) Clin. Chem. 41, 153–158; Zambon, A., Torres, A., Bijvoet, S., Gagne, C., Moojani, S., Lupien, P. J., Hayden M. R., and Brunzell, J. D. (1993) Lancet 341, 1119–1121). This may be due to an increase in the binding and uptake of lipoproteins by vascular tissue mediated by lipases (Eisenberg, S., Sehayek, E., Olivecrona, T. Vlodavsky, I. (1992) J. Clin. Invest. 90, 2013–2021; Tabas, I., Li, I., Brocia R. W., Xu, S. W., Swenson T. L. Williams, K. J. (1993) J. Biol. Chem. 268, 20419–20432; Nordestgaard, B. G., and Nielsen, A. G. (1994) Curr. Opin. Lipid. 5, 252–257; Williams, K. J., and Tabas, 1. (1995) Art. Thromb. and Vasc. Biol. 15, 551–561). Additionally, a high local level of lipase activity may result in cytotoxic levels of fatty acids and lysophosphatidylcholine being produced in precursors of atherosclerotic lesions.
Despite the understanding that has evolved regarding the role of lipase activity in lipid homeostasis, there nevertheless is a need in the art to identify additional genes coding for proteins that regulate lipid metabolism.